An example of a torsion bar having hexagonal heads is disclosed in document U.S. Pat. No. 4,231,555.
The ends of torsion bars of that type are generally formed in a single step by forging in a die which imparts their outside shapes to the hexagonal heads and to the shoulders, which die is constituted by a plurality of moving parts to enable the torsion bar to be removed after the hexagonal heads have been formed.
That method of making hexagonal heads can achieve only low dimensional precision for the heads.
In particular:
forging flash is generally present on the hexagonal heads and the shoulders in the join planes between the various moving parts of the die used for forging said heads; PA1 even ignoring the flash, the dimensions of the various portions of the hexagonal heads are not very precise; and PA1 the angular orientation of the hexagonal heads relative to each other about the central axis of the torsion bar is likewise not very precise. PA1 taking a torsion bar blank which includes said central body extending between said two ends; PA1 at each end of the torsion bar blank, forming the above-mentioned shoulder corresponding to said end together with a head blank by axially upsetting said end of the torsion bar blank while it is hot towards the central body in a hollow cylindrical die, said die not coming into contact with the shoulder and giving the head blank a cylindrical shape which contains more material than the hexagonal head that is to be formed; and PA1 machining each head blank by removing material until the corresponding hexagonal head has been obtained. PA1 during the upsetting of each end of the torsion bar blank, said end of the blank is heated by passing an electric current therethrough; PA1 the die is in the form of a single piece; PA1 the machining of the head blank by removing material is performed by milling; PA1 the hot upsetting of each end of the torsion bar blank is performed in a circularly cylindrical die giving the corresponding head blank a shape that is likewise circularly cylindrical; PA1 the torsion bar is a suspension torsion bar for a vehicle; PA1 the torsion bar is made of a carbon steel belonging to the 10xy category of the SAE J402 standard where x and y are two integers lying in the range 0 to 9; and PA1 the method also includes a step of surface quenching by induction.
Because of this poor dimensional precision, it can sometimes be difficult to incorporate torsion bars in the mechanical assemblies of which they are to form a part.
In particular, when torsion bars are to be fitted to vehicle suspensions, the poor precision concerning the relative orientation of the hexagonal heads makes it complicated and sometimes even impossible to perform initial adjustment of the various suspensions of any given vehicle so as to guarantee that the rest position of the vehicle is accurately horizontal. Said relative orientation of the hexagonal heads contributes to determining the rest height of the vehicle over each wheel.
In addition, forming the ends of the torsion bar by forging gives rise to structural defects (in particular concerning fiber orientation) that reduce the mechanical performance of the torsion bar in terms of the stresses that can be accepted, and that also reduce its lifetime.
To obtain satisfactory mechanical performance, it is therefore generally necessary to make torsion bars having hexagonal heads out of materials that are expensive, such as chromium steels or micro-alloyed steels.